This invention relates to the oxidation of hydrocarbons using a new family of compositions containing at least manganese and phosphate components in an extended network. These compositions can have a one-, two- or three-dimensional network and may be microporous. Further, the average manganese oxidation state varies from greater than 2.0 to 3.0.
Manganese occurs in a variety of oxidation states in its oxides, most notably +2, +3, and +4 in nature, and up to +7 in synthetic compounds. Because these different oxidation states are available to manganese, it is possible for manganese containing oxides to engage in oxidation chemistry, converting various compounds to more oxidized and often more useful forms. In order for manganese oxide systems to engage in oxidation chemistry, the average oxidation state of the manganese must be greater than +2, since the +2 oxidation state is the lowest stable oxidation state of Mn in its oxides. Hence, it is the compounds containing some manganese in the +3 and higher oxidation states that can engage in oxidation chemistry and catalysis. Manganese (IV) compounds are well known and are used in a variety of oxidation reactions. For example, manganese dioxide (MnO2) has been used in the manufacture of chlorine gas from hydrogen chloride and the oxidation of aniline to hydroquinone. See xe2x80x9cChemistry of the Elementsxe2x80x9d, N. N. Woodward and A. Earnshaw, Pergammon Press, Oxford, pp. 1219-20 (1984). A molecular manganese-oxo cluster is involved in the oxidation of water to oxygen in the photosynthesis process used by plants. See Yachandra et. al., Science, 260, 675-679 (1993). Because manganese has stable oxidation states of +4, +3 and +2, manganese oxides can be used in batteries.
Manganese oxides can have layered structures or three-dimensional microporous structures. S. Bach et al., Electrochimica Acta, 36, 1595-1603 (1991), P. LeGoff et al., Mat. Res. Bull., 31, 63-75 (1996), P. Strobel et al., Mat Res. Bull., 28, 93-100 (1993), Y. Shen et al., Science, 260, 511-515 (1993). Finally, the ion-exchange properties of manganese oxide compositions have been reported by Q. Feng et al. in Chem. Mater., 7,148-153 and 1722-1727 (1995).
A handful of Mn(III)-containing phosphate compounds are known, some occurring in nature, while others have been synthesized. Among the mineral phosphates containing Mn(III) are:
More information on Mn(III)-containing minerals, especially structure can be found in the review by Hawthorne in Z. Kristallogr., 192, 1 (1990). These materials fall in the classification of octahedral-tetrahedral framework structures, where Mn(III) is always found in octahedral coordination.
There are a number of examples of Mn(III)-containing phosphates that have been prepared by hydrothermal synthesis, e.g., KMn2O(PO4)(HPO4), Lightfoot et. al., J. Solid State Chem., 73, 325-329, (1988), and NH4Mn2O(PO4)(HPO4).H2O, Lightfoot et. al., J. Solid State Chem., 78, 17-22, (1989). These materials were obtained via hydrothermal transformation of Mn3O4 in the presence of KH2PO4 or NH4H2PO4 at 400xc2x0 C. and 220xc2x0 C. respectively, relatively harsh conditions. Similarly, MnPO4*H2O was prepared from Mn3O4, H3PO4, and water at 200xc2x0 C., Lightfoot et. al., Inorg. Chem., 26, 3544-3547, (1987). Solid-state ion-exchange of MnPO4*H2O with LiNO3 (4 weeks at 200xc2x0 C.) led to LiMn(PO4)(OH), Aranda et. al., Angew. Chem. Int Ed. Engl., 31, 1090-1092, (1992). A number of Mn(III) pyrophosphates, e.g., NH4MnP2O7, are known and have found use as pigments, Lee et. al., J. Chem. Soc. (A), 559-561, (1968). Mn(III) has also been substituted for up to one quarter of the VO3+ groups in the VOPO4*2H2O structure, forming [(Mn(H2O))x(VO)1xe2x88x92xPO4]*2 H2O, Richtrova et. al., J. Solid State Chem., 116, 400-405, (1995) . Finally, an example of a Mn(III)-phosphate complex is the water-soluble dipyridyl complex [Mn(III)(bpy)(HPO4) (H2PO4)]x, Sarneski et. al., Inorg. Chem., 32, 3265-3269, (1993).
In contrast to these references, applicant has synthesized crystalline manganese phosphate compounds which contain Mn(III) and which have an extended network. By extended network is meant that the defining Mnxe2x80x94Pxe2x80x94O structural unit of the material repeats itself into at least two adjacent unit cells without termination of bonding, i.e., the material is not molecular. See xe2x80x9cStructural Inorganic Chemistry, Fifth Edition,xe2x80x9d A. F. Wells, Clarendon Press, Oxford, pp. 11-15, (1984). The network can be one-dimensional (a linear chain), two-dimensional (layered) or three-dimensional. The three dimensional network may or may not be a microporous network. By Mn(III)-containing phosphate, it is meant that the average oxidation state of Mn is greater than 2.0 but less than or equal to 3.0, indicating the presence of some Mn(III). These compositions are prepared by trapping the desired manganese oxidation state via titrimetric methods, reduction of a novel manganese(IV) phosphate solution, or hydrothermal transformations of birnessite-like (e.g., Na4Mn14O27*xH2O) materials, all in the presence of excess phosphate at specific reaction conditions. Besides MnPO4*H2O, which is ubiquitous in Mn(III) phosphate chemistry and not part of this invention, these methods have not yielded any of the synthesized compounds or mineral structures noted above. This may be due to the mild conditions and the higher reactivity of the reagents employed. In addition, applicant discloses the first manganese(III) and mixed valence Mn(III)/Mn(III) phosphates containing organoammonium cations. Further, applicant has also synthesized metallo manganese phosphates where a portion of the manganese is replaced by a metal such as iron (III), aluminum, gallium, etc.
This invention relates to hydrocarbon oxidation processes using a crystalline Mn(III)-containing phosphate composition having an extended network. Accordingly, one embodiment of the invention is a process for the oxidation of hydrocarbons comprising contacting a hydrocarbon with a crystalline manganese phosphate composition in the presence of oxygen under oxidation conditions to give an oxidized product, the manganese phosphate composition having an extended network and an empirical composition on an anhydrous basis expressed by an empirical formula of:
(Aa+)v(Mnb+)(Mc+)xPyOz
where A is a structure directing agent which balances the charge on the manganese phosphate framework and is selected from the group consisting of alkali metals, alkaline earth metals (except calcium), hydronium ion, ammonium ion, organoammonium ions, silver, copper (II), zinc (II), nickel (II), mercury (II), cadmium (II) and mixtures thereof, xe2x80x9caxe2x80x9d represents a weighted average valence of A and varies from about 1.0 to about 2.0, xe2x80x9cvxe2x80x9d is the mole ratio of A to Mn and varies from about 0.1 to about 10.0, xe2x80x9cbxe2x80x9d is the average valence of Mn and has a value of greater than 2 to a maximum of 3, M is a metal selected from the group consisting of Al, Fe3+, Ga, Sn4+, Ti, Sb5+, Ag, Zn, Cu, Ni, Cd, and mixtures thereof, xe2x80x9cxxe2x80x9d is the mole ratio of M to Mn and varies from 0 to about 3.0, xe2x80x9ccxe2x80x9d is the weighted average valence of M species and varies from about 1.0 to about 5.0, xe2x80x9cyxe2x80x9d is the mole ratio of P to Mn and varies from about 0.05 to about 8.0 and xe2x80x9czxe2x80x9d is the mole ratio of O to Mn and has a value determined by the equation
z=xc2xd((axc2x7v)+b+(xxc2x7c)+(5xc2x7y)).
This and other objects and embodiments of the invention will become more apparent after a more detailed description of the invention.
Oxidation of hydrocarbons involves contacting a hydrocarbon stream with the manganese phosphate compositions described herein at oxidation conditions. Non-limiting examples of these processes include conversion of hydrocarbons to alcohols, ethers, aldehydes, ketones or acid anhydrides; conversion of paraffins to olefins, ammoxidation of paraffins and dimerization of olefins, oxidative dehydrogenation of paraffins, oxidative dehydrodimerization of olefins and ammoxidation of paraffins are commercially more important and will be described in more detail.
When the process is oxidative dehydrodimerization (hereinafter dimerization), the process involves contacting an olefin stream with oxygen and the manganese phosphate composition described hereinafter at dimerization conditions. The manganese phosphate can be used in any configuration, but is usually present as a fixed bed or a radial flow bed. In a fixed bed configuration, the olefin stream can be flowed upflow or downflow although not necessarily with equivalent results. The olefin stream, oxygen and manganese phosphate are contacted for a time sufficient to form dimerized product. In terms of weight hourly space velocity (WHSV) the contact time can vary from about 1 to about 50 hrxe2x88x921. Other dimerization conditions include a temperature of about 350xc2x0 C. to about 650xc2x0 C., preferably from about 450xc2x0 C. to about 600xc2x0 C., and a pressure from about 0 to about 1000 psig. It is also necessary that oxygen be present during the reaction. The source of oxygen can be either pure oxygen, air, oxygen with an inert diluent gas such as helium or lattice oxygen from the manganese phosphate. The amount of oxygen present is such that the ratio of olefin to oxygen (O2) varies from about 1.5:1 to about 10:1 preferably from 3:1 to 8:1.
The oxidative dehydrodimerization can also be carried out using the lattice oxygen of manganese phosphate as the source of oxygen. This approach can often yield higher selectivities to dimers than can be obtained with the oxygen co-fed process described above. In this mode of operation, manganese is reduced when exposed to the olefin feed. When a portion of the available lattice oxygen is used a regeneration step is carried out to restore the oxidation capability of the manganese phosphate. The manganese phosphate composition can be cycled between oxidation and regeneration numerous times.
For oxidative dehydrogenation, the manganese phosphate composition can be used in the same manner as described above for dimerization. Contacting of the paraffin with oxygen and the catalyst is done for a sufficient time to dehydrogenate the paraffin. Accordingly, the GHSV varies from about 100 to about 10,000 hrxe2x88x921 and preferably from about 300 to about 2000 hrxe2x88x921. Other dehydrogenation conditions include a temperature of about 250xc2x0 C. to about 650xc2x0 C. and a pressure of about 1 to about 45 psig. The source of oxygen can be any of the ones enumerated above and is present in a ratio of paraffin:O2 of about 1:10 to about 5:1.
Ammoxidation of paraffins is carried out in a similar manner to the processes above with the following differences. The process involves reacting a paraffin with ammonia in the presence of a manganese phosphate composition. Ammoxidation conditions include a GHSV of about 100 to about 5,000 hr1, a temperature of about 300xc2x0 C. to about 520xc2x0 C. and a pressure of about 1 to about 45 psig. Finally, the ammonia: paraffin ratio varies from about 1:10 to about 5:1.
Ammoxidation of paraffins is carried out in a similar manner to the processes described above with the following differences. The process involves reacting a paraffin with ammonia in the presence of a manganese phosphate composition. Ammoxidation conditions include a GHSV of about 100 to about 5,000 hrxe2x88x921, a temperature of about 300xc2x0 C. to about 520xc2x0 C. and a pressure of about 1 to about 45 psig. Finally, the ammonia: paraffin ration varies from about 1:10 to about 5:1. Surprisingly, in certain instances, this process resulted in the conversion of ammonia to nitrous oxide N2O. Nitrous oxide is a valuable oxidant and specialty chemical.
The manganese phosphate compositions used in the processes described above have a chemical composition on an anhydrous basis expressed by the empirical formula:
(Aa+)v(Mnb+)(Mc+)xPyOz
where A is a templating agent selected from the group consisting of alkali metals, alkaline earth metals (except calcium), hydronium ion, ammonium ion, organoammonium ions, silver, copper (II), zinc (II), nickel (II), mercury (II), cadmium (II), and mixtures thereof, xe2x80x9caxe2x80x9d represents a weighted average valence of A and varies from about 1.0 to about 2.0, xe2x80x9cvxe2x80x9d is the mole ratio of A to Mn and varies from about 0.1 to about 10, xe2x80x9cbxe2x80x9d is the average valence of Mn and has a value of greater than 2 to a maximum of 3, M is a metal selected from the group consisting of Al, Fe3+, Ga, Sn4+, Ti, Sb5+, Ag, Zn, Cu, Ni, Cd, and mixtures thereof, xe2x80x9cxxe2x80x9d is the mole ratio of M to Mn and varies from 0.0 to about 3.0, xe2x80x9ccxe2x80x9d is the weighted average valence of M and varies from about 1.0 to about 5.0, xe2x80x9cyxe2x80x9d is the mole ratio of P to Mn and varies from about 0.05 to about 8.0 and xe2x80x9czxe2x80x9d is the mole ratio of O to Mn and has a value determined by the equation
z=xc2xd((axc2x7v)+b+(xxc2x7c)+(5xc2x7y)).
The alkali metals include lithium, sodium, potassium, rubidium and cesium, while the alkaline earth metals include magnesium, strontium and barium. Illustrative examples of organoammonium ions include but are not limited to methylammonium, ethylenediammonium, propylammonium, and hexylammonium.
When A is one structure directing agent, the weighted average valence is the valence of the one structure directing agent. However, when more than one templating agent is used, the total amount of:       A    v          a      +        =            A      i                        a          i                +              +          A      j                        a          j                +              +          A      k                        a          k                +              +    …  
and the weighted average valence xe2x80x9caxe2x80x9d is defined by:   a  =                              a          i                ·        i            +                        a          j                ·        j            +                        a          k                ·        k            +      …              i      +      j      +      k      +      …      
The weighted average valence of manganese (xe2x80x9cbxe2x80x9d) is dependent on the amount of Mn2+ and Mn3+ present in the composition. Thus, if the total amount of manganese xe2x80x9cwxe2x80x9d is defined by w=p+q, where xe2x80x9cpxe2x80x9d is the mole fraction of Mn2+ , xe2x80x9cqxe2x80x9d is the mole fraction of Mn3+ then the average valence:   b  =                              2          ⁢          p                +                  3          ⁢          q                            p        +        q              .  
Similarly, when two or more metals (M) are present, the amount of each metal is defined by:       M    x          c      +        =            M      α              c        α        +              +          M      β                        c          β                +              +          M      γ                        c          γ                +              +    …  
and the average valence C is determined by the equation:   C  =                              C          α                ·        α            +                        C          β                ·        β            +                        C          γ                ·        γ            +      …              α      +      β      +      γ      +      …      
The crystalline compositions of the invention are characterized in that they have an extended network. By extended network is meant that the defining Mnxe2x80x94Pxe2x80x94O structural unit of the material repeats itself into at least two adjacent unit cells without termination of bonding, i.e., the material is not molecular. See xe2x80x9cStructural Inorganic Chemistry, Fifth Edition,xe2x80x9d A. F. Wells, Clarendon Press, Oxford, pp. 11-15, (1984). The compositions can have a one-dimensional network which is a linear chain, a two-dimensional network which is a layered network or a three-dimensional network which is either a microporous framework structure or a non-microporous framework structure.
The instant manganese phosphate compositions are prepared by hydrothermal crystallization of a reaction mixture prepared by combining reactive sources of phosphorus, manganese, optionally one M metal, at least one structure directing agent (A) plus water. Specific examples of the phosphorus sources which can be used in this invention are orthophosphoric acid, pyrophosphoric acid, alkali phosphates and sodium metaphosphate. The manganese source can be either a high oxidation state salt such as KMnO4, NaMnO4, CsMnO4, NH4MnO4, Mg(MnO4)2 and Ba(MnO4)2. Other sources of manganese are birnessite and buserite. Birnessite, e.g., Na4Mn14O27.9H2O, and a more hydrated form called buserite are layered manganese oxides which contain charge balancing cations and water between the layers. The oxidation state of Mn in these materials is about +3.3 to about +3.7. These materials can be prepared in a variety of cation forms such as Li+, Na+, K+, Rb+, Cs+ and NH4+ where the cations are present between the layers. These cations can be partially or totally exchanged with other cations such as Mg2+, Sr2+, Ba2+, Zn2+, Cu2+, Ag+, Ni2+, Cd2+, Hg2+, or mixtures thereof.
The layers can also be expanded with organoammonium cations via an initial treatment in acid followed by a treatment with an amine. A variety of amines including propylamine may be employed for the purpose of creating reactive expanded birnessites for use in making the new manganese phosphates of this invention. More details on preparing expanded birnessites can be found in Wong and Cheng, Inorg. Chem., 31, 1165-1172. These sources of manganese oxide are very reactive and easily transformed to other structures. The Mg-exchanged version of bimessite can be structurally transformed hydrothermally to form the microporous manganese oxide todorokite, Y. Shen et al., Science, 260, 511-515 (1993). In this invention, the various forms of birnessite and buserite are hydrothermally transformed in the presence of excess phosphate, where phosphate can engage in bonding with the manganese oxide while structural transformations occur. The oxidation state of Mn in the resulting compounds is usually +3 or less, depending on pH, temperature, and the nature of A.
Manganese sources can also be any layered manganese oxide pillared with a variety of organoammonium cations such as methylammonium, ethylammonium, propylammonium, butylammonium, hexylammonium, ethylenediammonium, tetramethylammonium and mixtures thereof. Additionally, 1,4 diazabicyclo[2.2.2] octane (DABCO) alone or in combination with organoammonium cations such as enumerated above can also be used. Combinations of ammonium cation and any organoammonium cations (and/or DABCO) can also be used. The series of layered manganese (IV)-containing phosphates, examples of which are described as NaMnP-1a and NaMnP-1b in U.S. Pat. No. 5,780,003, (which is incorporated by reference) in their various metal substituted (for manganese) and cation exchanged forms, also can be used as sources of manganese for preparing the manganese phosphates of this invention.
Finally, it is preferred to use a special manganese solution prepared from NaMnO4 and H3PO4 whose preparation is described in detail in example 1 of U.S. Pat. No. 5,780,003, which is incorporated by reference. This phosphate-stabilized manganese solution has a composition represented by the empirical formula
NaMnO4:rH3PO4:sR:uH2O
where R is a reductant selected from the group consisting of H2C2O4,Na2C2O4, NaHCO2 and Mn(NO3)2.6H2O, xe2x80x9crxe2x80x9d has a value of about 3.0 to about 30, xe2x80x9csxe2x80x9d is the mole ratio of R:NaMnO4 sufficient to reduce the manganese oxidation state to a value of greater than 3 to about 4 and varies from about 1.5 to about 4, and xe2x80x9cuxe2x80x9d is the moles of water and varies from about 25 to about 1000 in order to vary the manganese concentration from 0.1 wt. % to about 5 wt. %. The preferred form of this solution contains 1 wt. % manganese and is called the xe2x80x9c1% Solutionxe2x80x9d in the rest of this application. The advantage to this solution is that it is stable over a long period of time, i.e., months, and facilitates the preparation of the instant compositions by adding a templating agent, such as an organoammonium salt or amine, or alternatively an alkali metal or alkaline earth in combination with a suitable portion of reductant to this solution and heating the resultant mixture. This solution is especially convenient with the organoammonium cations and amines because these structure directing agents react uncontrollably with other soluble high oxidation state Mn salts, such as the permanganates (MnO4xe2x88x92) listed above. Substituent metals, such as Al, Fe, and others may also be introduced into this solution and aged before the appropriate crystallization inducing agents are added. Using this solution and suitable reductants, it is possible to prepare a variety of Mn (III) and mixed valence Mn(III)/Mn(II) phosphates. Examples of reductants include but are not limited to amines and organoammonium species, which also serves as the charge-balancing species for the manganese phosphate framework. In this process, the original amine or organoammonium species will be partially oxidized, often resulting in the formation of fragments of the original amine or organoammonium species. Hence, the charge-balancing species for the manganese phosphate framework may be one of these fragments or a mixture thereof and different from the original amine or organoammonium species introduced into the synthesis mixture. Because of this, the elemental analyses will often yield non-integral carbon/nitrogen ratios for the organoammonium groups. Other reductants include inorganic or organoammonium formates and oxalates and various chloride salts.
Another method to prepare the manganese phosphates of this invention is via the hydrothermal treatment of MnPO4*H2O or its metal-substituted forms in alkaline solutions. The alkaline solutions may be formed by alkaline hydroxides, organoammonium hydroxides, ammonium hydroxide, or a variety of amines. The manganese(III) phosphate MnPO4*H2O used for these reactions is easily prepared by reacting excess concentrated phosphoric acid with nitric acid and Mn (NO3)2.4H2O and isolating the MnPO4.H2O product.
Yet another way to make the novel manganese phosphates of this invention is to use a xe2x80x9ctrappingxe2x80x9d method in which the desired manganese oxidation state is generated in the presence of excess phosphate via a redox reaction. For example, a soluble permanganate MnO4xe2x88x92 is dissolved in a solution containing excess phosphoric acid and is reduced using a reducing agent such as formate or oxalate salts. The purpose of the excess phosphate is to avoid the formation of insoluble manganese oxides in the +3 and +4 oxidation states and insure that a manganese phosphate forms instead. Hence, as the typically insoluble oxidation states of manganese are formed, they are xe2x80x9ctrappedxe2x80x9d and stabilized by the excess phosphate.
The non-manganese sources must be chosen in a manner such that the source, the digestion temperature, and the pH will yield manganese phosphates with average oxidation states greater than 2+ , but less than or equal to 3.0. Not every source of structure directing agent enumerated in the following list is compatible with every source of manganese with respect to attaining the desired manganese oxidation state. The source of the alkali or alkaline earth metals structure directing agents include the acetate, nitrate, carbonate, and hydroxide compounds. Specific examples include sodium chloride, sodium nitrate, sodium acetate, sodium carbonate, sodium hydroxide, lithium chloride, lithium nitrate, lithium carbonate, lithium hydroxide, rubidium chloride, rubidium nitrate, rubidium carbonate, rubidium hydroxide, cesium chloride, cesium nitrate, cesium carbonate, cesium hydroxide, potassium chloride, potassium nitrate, potassium carbonate, potassium hydroxide, magnesium chloride, magnesium nitrate, magnesium carbonate, magnesium hydroxide, barium chloride, barium nitrate, barium carbonate, barium hydroxide, strontium chloride, strontium nitrate, strontium carbonate and strontium hydroxide. Sources of organoammonium ions include methylamine, hexylamine, propylamine, and ethylenediamine. The organoammonium cation is generated in situ via protonation. Organoammonium cations may also be quaternized, such as tetramethylammonium and tetraethylammonium, employed as either hydroxides or chlorides. Finally, sources of the M metal include the nitrate salts of the metals as well as TiCl3, NaSbF6, and SnCl4.
Generally, the hydrothermal process used to prepare the manganese phosphate of this invention involves forming a reaction mixture which has the formula:
dAOa/2:MnOm/2:eMOc/2:fP2O5:gB:hR:tH2O
where B is a mineralizer, R is a reductant, xe2x80x9cdxe2x80x9d ranges from about 0.5 to about 20, xe2x80x9cexe2x80x9d ranges from 0 to about 3.0, xe2x80x9cfxe2x80x9d ranges from about 0.5 to about 15, xe2x80x9cgxe2x80x9d ranges from 0 to about 2, xe2x80x9chxe2x80x9d ranges from 0 to about 5, xe2x80x9ctxe2x80x9d ranges from about 25 to about 1000, and xe2x80x9cmxe2x80x9d ranges from about 3 to about 7. Examples of the mineralizer B include HF and NaF, while examples of the reductant R include NaHCO2, H2C2O4, and Na2C2O4.
It also is necessary to adjust the pH of the mixture to a value of about 2.0 to about 12.0. The pH of the mixture can be controlled by addition of a base such as NaOH, NH4OH, amines, etc.
Having formed the reaction mixture, it is next reacted at a temperature of about 50xc2x0 C. to about 175xc2x0 C. for a period of about 12 hours to about 240 hours. The reaction is carried out under atmospheric pressure or the reaction vessel may be sealed and the reaction run at autogenous pressure. In a preferred method the phosphorus source and the manganese source is the xe2x80x9c1% solutionxe2x80x9d, the temperature is from about 70xc2x0 C. to about 100xc2x0 C. and the time required to crystallize the product is from about 16 hours to about 120 hours.
It should be pointed out that not all the enumerated structure directing agents can provide all the various structures possible in the generic class of extended network manganese phosphate compositions. The relationship of specific structure directing agents to individual products is apparent from the illustrative examples set forth herein.
The oxidation state of manganese in the manganese phosphates described here is one of the characterizing properties of these new materials. The measurement of the oxidation state of manganese was carried out according to a variation of the oxalate method given in Piper et. al., Geochimica et Cosmochimica Acta, 48, 1237-1247, (1984). The Mn-containing sample is reduced to Mn2+ when it is digested at 85xc2x0 C. in a dilute sulfuric acid solution containing a known excess of sodium oxalate, the reducing agent. The solution is divided into two portions, one of which is analyzed for total Mn. In the second portion, the excess oxalate not consumed by the reduction of the sample is back-titrated with standardized KMnO4, allowing the determination of the amount of oxalate consumed by the sample. The average oxidation state of manganese is then determined from the amount of oxalate consumed and the concentration of Mn in the sample.
In the examples which follow elemental analyses were conducted on air dried samples. Analysis was carried out for all elements except oxygen. Organoammonium and ammonium salts were determined by high temperature oxidative pyrolysis, yielding C, H, and N analyses. Because of the oxidizing nature of the reaction mixture used to prepare the compositions of this invention, all the metals (other than Mn) were assumed to be in their highest oxidation state, e.g., Fe3+ or Ti4+. Therefore, the oxygen stoichiometry was determined from the known oxygen requirements of all of the elements including the measured oxidation state of the manganese.
The structure of the manganese phosphates of this invention was determined by x-ray analysis. The x-ray patterns presented in the following examples were obtained using standard x-ray powder diffraction techniques. The radiation source was a high-intensity, x-ray tube operated at 45 kV and 35 ma. The diffraction pattern from the copper K-alpha radiation was obtained by appropriate computer based techniques. Flat compressed powder samples were continuously scanned at 2xc2x0 (2xcex8) per minute from 2xc2x0 to 70xc2x0 (2xcex8). Interplanar spacings (d) in Angstrom units were obtained from the position of the diffraction peaks expressed as xcex8 where xcex8 is the Bragg angle as observed from digitized data. Intensities were determined from the integrated area of diffraction peaks after subtracting background, xe2x80x9cIoxe2x80x9d being the intensity of the strongest line or peak, and xe2x80x9cIxe2x80x9d being the intensity of each of the other peaks.
As will be understood by those skilled in the art the determination of the parameter 20 is subject to both human and mechanical error, which in combination can impose an uncertainty of about xc2x10.4xc2x0 on each reported value of 2xcex8. This uncertainty is, of course, also manifested in the reported values of the d-spacings, which are calculated from the 2xcex8 values. This imprecision is general throughout the art and is not sufficient to preclude the differentiation of the present crystalline materials from each other and from the compositions of the prior art. In some of the x-ray patterns reported, the relative intensities of the d-spacings are indicated by the notations vs, s, m, and w which represent very strong, strong, medium, and weak, respectively. In terms of 100xc3x97I/Io, the above designations are defined as:
w=0-15; m=15-60; s=60-80 and vs=80-100.
In certain instances the purity of a synthesized product may be assessed with reference to its x-ray powder diffraction pattern. Thus, for example, if a sample is stated to be pure, it is intended only that the x-ray pattern of the sample is free of lines attributable to crystalline impurities, not that there are no amorphous materials present.
To allow for ready reference, the different structure types in the following examples have been given arbitrary numbers such as MnP-1. Thus NH4MnP-7 and RbMnP-7 have the same structure, i.e., structure type 7. Additionally, variations have been observed in compositions having the same structure types. These have been designated by a letter after the number, e.g., MnP-14a and MnP-14b. The crystalline compositions of the instant invention may be characterized by their X-ray powder diffraction patterns and such may have one of the X-ray patterns containing the d-spacings and intensities set forth in the following tables. The intensities are the relative intensities as stated above.